A METHOD AND AN APPARATUS FOR HANDING OFF MOBILE STATION FROM A FIRST TO A SECOND CHANNEL IN A MOBILE COMMUNICATION SYSTEM

Abstract: In a mobile communication system, a method and an apparatus for handing off a mobile station (MS) from a first channel (CH1) on a serving base station (BS1) to a second channel (CH2) on a target base station (BS2), said mobile station (MS) transmitting at a first transmission phase on said first channel (CH1), said second channel (CH2) being ready to take over the communication with said mobile station (MS) transmitting at a second transmission phase, wherein a time offset determining means (TODM) is provided in said target base station (BS2) and is adapted to tune in to said mobile station (MS) for determining, prior to a handoff, the time offset between said first transmission phase and said second transmission phase, and a time shifting means (TSM) is provided for shifting, upon handoff, the transmission phase of said mobile station (MS) by said determined time offset relative to said first transmission phase.

Description: A METHOD AND AN APPARATUS FOR HANDING OFF MOBILE STATION FROM A FIRST TO A SECOND CHANNEL IN A MOBILE COMMUNICATION SYSTEM TECHNICAL FIELD The invention relates to a method for handing off a mobile station from a first channel on a serving base station to a second channel on a target base station in a mobile communi¬ cation system. The method also includes handing off the mobile station from the first channel to the second channel in the serving base station. The mobile station is transmitting at a first transmission phase on said first channel, said second channel being ready to take over the communication with said mobile station transmitting at a second transmission phase. The invention also relates to an apparatus for carrying this method into effect. BACKGROUND OF THE INVENTION In a mobile communication system, e.g. a cellular mobile radio system, an inter-cell handoff is the actual switching of an ongoing call from a base station in one cell to a base station in another cell, while an intra-cell handoff is the actual switching of a connection from one channel to another channel in one and the same base station. Inter-cell handoffs are performed in order to effect a change of base station, e.g. due to the mobile station moving in a cellular environment, or to relocate some traffic from a highly loaded base station to a less loaded base station. An intra-cell handoff takes place e.g. if the quality on the currently used channel falls below the minimum accepted qua- lity or during a call set-up procedure when handing off the mobile station from a control channel to a traffic channel. When an inter-cell or intra-cell handoff is performed bet¬ ween unsynchronized channels or between channels having a phase difference unknown to the mobile station, the mobile station must resynchronize itself on to the new channel. This resynchronization process takes considerable time, e.g. 200 ms, resulting in a non-negligible interruption of the speech signal which can cause disturbing interruptions in the ongoing speech. The patent application WO 9222966 shows a method in a cellular mobile radio telephone system for initially setting a time alignment parameter in a mobile station. The method comprises the steps of estimating an optimal time alignment parameter for use during communication on a new traffic channel of a desired base station, sending the estimated time alignment parameter to the mobile station, and setting the time alignment value in the mobile station. The patent application EP 0430106 A2 shows a method for hand over i a mobile telephone system, by using mobile stations with two local oscillators. By having two local oscillators it is possible to receive on a first channel and at the same time trans itt on another channel. This can be used to reduce the time duration of making a hand over. DISCLOSURE OF INVENTION The object of the invention is to bring about a method and an apparatus for speeding up the process of handoff between base stations. A further object is to speed up the set up of a connection by speeding up the handoff from a control channel to a traffic channel. A still further object is to speed up handoff between traffic channels in a common base station, to avoid interruptions in the communication in a handoff process, and to avoid the process of resynchronization. This is attained by the method according to the invention mainly by, prior to a handoff, determining, by tuning in to said mobile station in said target base station, the time offset between said first transmission phase and said second transmission phase, and, upon handoff, shifting the transmission phase of said mobile station by said determined time offset relative to said first transmission phase. This is also attained by the apparatus according to the invention which comprises a time offset determining means provided in said target base station and adapted to tune in to said mobile station for determining, prior to a handoff, the relative time offset between said first transmission phase and said second transmission phase, and a time shif¬ ting means for shifting, upon handoff, the transmission phase of said mobile station by said determined time offset relative to said first transmission phase. Examples of mobile communication systems where the invention can be used are systems utilising frequency division (FDMA) , time division (TDMA) , coding (CDMA) and the so called DECT- system. One of the advantages with the inventionen is that the process of resyncronization is avoided, resulting in a reduction in calculation, which saves computer capacity. Another advantage is that the speech quality is improved, due to that an interruption of the speech channel is avoided. BRIEF DESCRIPTION OF DRAWING The invention will be described more in detail below with reference to the appended drawings, on which Fig. 1 is a schematical illustration of a mobile radio sys- tem, Figs. 2a - 2e show exemplary different transmission phases occuring during a handoff in the mobile radio system according to Fig. 1. Fig. 3 shows a diagram, in which signal strength is used to determine when to make a handoff. Fig. 4 show two block diagrams illustrating a time shifting means and a time offset determining means, and Fig. 5 shows a flow chart illustrating a method for speeding up the process of handoff according to the invention. PREFERRED MODES FOR CARRYING OUT THE INVENTION Fig. 1 is a schematical illustration of a mobile radio sys¬ tem in which the method and the apparatus according to the invention are utilized. The mobile radio system shown in fig. 1 comprises two base stations BSl and BS2 which are located in adjacent cells Cl and C2. The base stations BSl and BS2 are connected e.g. by cables to a mobile services switching centre MSC or any other type of switching/controlling unit, such as a base station controller (BSC) , a mobile telephone switching office (MTSO) , etc., used in cellular communication. The mobile services switching centre MSC is connected e.g. by cables to a fixed public switched telephone network PSTN also in a manner known per se. It is to be understood that many more base stations than shown may be connected to the mobile services switching centre MSC. In Fig. 1, a single mobile station MS is shown. However, it should be understood that many more mobile stations may be present in the mobile radio system according to Fig. 1. The mobile station MS is supposed to be communicating with the currently serving base station BSl on a digital traffic channel CHI and transmitting at a transmission phase TO as shown in Fig. 2a. In fig. 2a-e the time is denoted by r. The transmission of the mobile station MS is received, delayed due to a propagation delay dl, in the base station BSl at a transmission phase Tl as shown in Fig. 2c. The base station BSl supervises the channel CHI by monitor¬ ing e.g. the signal strength and/or the bit error rate. Fig. 3 shows a diagram over the variation of the signal strength SS from the base stations BSl and BS2 with the position of the mobile station MS from the base station BSl. The mobile station monitors the quality (e.g. signal stre¬ ngth) of the channel CHI belonging to the base station BSl and the quality of the channel CH2 belonging to the base station BS2. The distance between the mobile station MS and the base station BSl is denoted by X. A curve SI shows the signal strength from the base station BSl and a curve S2 shows the signal strength from the base station BS2. The mobile station MS has contact via the traffic channel CHI with the base station BSl and moves in direction towards the base station BS2. In a first position XI the signal strength from the base station BSl is Sll, and the signal strength from the base station BS2 is S21. Since the signal strength Sll is greater than the signal strength S21 the communication with the base station BSl continues. In a second position X2 the signal strength S12 from the base station BSl is less than the signal strength S22 from the base station BS2, resulting in a handoff from the base station BSl to the base station BS2. The mobile station MS measures regulary the signal strength from the base stations and reports the results from the measurements via a control channel, for exampel the SACCH channel in the GSM-system, to a the mobile communication system. The results can for example be reported to the base stations BSl and BS2 via a control channel or to any other type of switching/controlling unit, such as a base station controller (BSC) , a mobile telephone switching office (MTSO) , etc., used in cellular communica- tion. It should be obviuos to one of ordinary skill in the art, that the mobile station MS can monitor the quality of the channel CHI in other ways, e.g. by measuring the bit error rate (BER) instead of the signal strength in a similar way as is described above. When the mobile station MS is connected via the channel CHI, the mobile station MS monitors the quality of the channel CHI, e.g. by measuring the bit error rate and/or the recei- ved signal strength Sll on the channel CHI. In addition, the received signal strength and/or the bit error rate on some given channels, e.g. channel CH2 located in neighbouring cells should be measured, for example the signal strength S21 from the base station BS2 in the cell C2. Such measure- ments are initiated in the mobile station MS by e.g. the base station BSl. The measurements of the channels are compared e.g. in the base station BSl or the mobile services switching centre MSC by looking at the received signal strengths and/or bit error rates BER reported by the mobile station MS via control channels. If the comparison indicates that the connection is better served on another channel on the same base station BSl or by another base station, e.g. base station BS2, the base station BSl generates a handoff request to the mobile services switching centre MSC. In the latter case, the base station BS2, in its role as target base station, will be instructed to take over the communication with the mobile station MS on a channel CH2. In an alternative preferred embodiment, base stations neigh¬ bouring the currently serving base station BSl, tune to the channel CHI currently used by the mobile station MS and measures the received signal strength. The results of the measurements are reported to the mobile services switching centre MSC. If a comparison of the results indicates that the connection is better served by another base station, a handoff to this base station will be performed. In Fig. 2a-e the square puls indicates a burst in a TDMA- (Time Division Multiple Access)-system, even though the figures are representative for other systems as well, such as systems utilising frequency division (FDMA) , coding (CDMA) and the so called DECT-system. The desired transmission phase T2 of the mobile station MS on the channel CH2 is, as shown in Fig. 2b, displaced in time by a time offset T from the transmission phase TO of the mobile station MS on the channel CHI, shown in Fig. 2a. The transmission phase T3 of the mobile station MS on channel CHI as received, delayed due to a propagation delay d2, in the base station BS2 is shown in Fig. 2d. Shown in Fig. 2e is the transmission phase T4 of the mobile station MS as desired to be received, delayed due to the propagation delay d2 , in the base station BS2. As shown, the time offset T between the desired transmission phase of the mobile station MS on the channel CH2, Fig. 2b, and the transmission phase of the mobile station MS on chan¬ nel CHI, Fig. 2a, is the same as the time offset T between the transmission phase of the mobile station MS on the channel CH2 as desired to be received in the base station BS2, Fig. 2e, and the transmission phase, as received in the base station BS2, of the mobile station MS on the channel CHI. According to the invention, the time offset T between the transmission phase of the mobile station MS communicating on the channel CHI, as received in the base station BS2, Fig. 2d, and the desired transmission phase of the mobile station MS to be correctly received in the base station BS2 on the channel CH2, Fig. 2e, is determined in the base station BS2 by a time offset determining means TODM adapted to tune in to the mobile station MS. The time offset determining means TODM will be explained in the description of Fig. 4. Prior to a handoff, the mobile station MS is informed about this time offset T or provided with information allowing the mobile station MS to establish the time offset T, e.g. in that the base station BS2 informs the mobile services switc- hing centre MSC, which in its turn informs the base station BSl which in its turn transmits this information about the time offset to the mobile station MS. This information can be sent by using a control channel, e.g. the SACCH channel. Upon handoff, i.e. when the mobile station MS is to start communicating with the base station BS2, the mobile station MS is instructed to shift its transmission phase TO according to Fig. 2a by the determined time offset T to fit the transmission phase T2 according to Fig. 2b on channel CH2. To accomplish this the mobile station MS is provided with a time shifting means TSM adapted to shift the transmission phase of the mobile station MS by the determined time offset T. The time shifting means TSM will be explained below in the description of Fig. 4. Hereby, the transmission phase of the mobile station MS will be adapted to the correct transmission phase on the channel CH2 without any need of resynchronization prior to the initial transmission on channel CH2. Fig. 4 shows the time offset determining means TODM, comprising a correlator 10, an apparatus for time measurements 12, and an apparatus for time calculation 14. The correlator 10 obtains the mobile stations MS syncronization word 16 from the mobile communication system and receives the demodulated data 24 from the mobile station MS. At its output the correlator 10 generates correlator trig pulses 18, which are received by the apparatus 12 for time measurements, which also receives frame timing trig pulses 26 corresponding to the timing of the target channel CH2. The apparatus 12 generates at its output a time value 20, indicating the time between frame timing trig pulses 26 and correlator trig pulses 18. This time value 20 is received by the apparatus 14 for time calculation, in which the calculation of the time offset is done, and on which output the time offset 22 is generated. The time offset 22 is essantially identical to the time offset T in fig. 2. When the target base station BS2 shall verify the mobile station MS it gets the syncronization word from the mobile communication system. Fig. 4 also shows the time shifting means TSM, comprising a trig generator 2, which generator 2 receives an internal clock signal 8 from the mobile station MS and an offset value 6. The offset value 6 depends on the calculated time offset 22 in Fig. 4 described above. The trig generator 2 generates at its output start burst trig pulses 4. By changing the offset value 6 the timing of the trig pulses, i.e. the phase of the pulse train, is shifted, and hence the start burst trig pulses 4 can be controlled. The start burst trig pulses 4 is used to initiate and syncronize the transmission from the mobile station MS. The time shift means TSM is in this embodiment located in the mobile station MS, but it can also be located elsewhere, e.g. in the base stations or in the mobile services switching centre MSC. The time offset determining means TODM is located in the target base station BS2, but can also be located elsewhere in the mobile communication system, e.g. in the mobile station MS. Syncronization word, time offset, frames and other expres- sions used in the description of Fig. 4 are well known expressions for those ordinary skilled in the art and are described in the standard TIA-IS-54. Fig. 5 shows a flow chart illustrating the method for speeding up the process of handoff according to the invention. The base station BSl is serving the mobile station MS and the basestation BS2 is acting as target base station, in accordance with Fig. 1. The flow charts starts at step 46. Next, at step 30 the mobile communication system detects the need for handing off the mobile station MS from the serving base station BSl to a target base station BS2. The detection can be done by measuring the signal strength as has been explained in the description to Fig. 3 above. At step 32 the system requests the target base station BS2 to verify the mobile station MS, which is a well known procedure. During the verification the target base station BS2 tunes at step 34 to the frequency of the mobile station MS and locks onto the syncronization word. At step 34 the target base station BS2 can also optionally verify the identification of the mobile station MS. The flow chart continues with step 36, in which the target base station BS2 determines how much in time the mobile station MS must shift its frame timing in order to fit the frame timing of the new channel served by the target base station BS2. Step 36 is performed in the time offset determining means TODM as described above in the description to Fig. 4. After Step 36 follows step 38, in which the target base station BS2 reports the time offset T determined at step 36, to the mobile communication system, e.g. to the mobile services switching centre MSC or in a GSM-system to a base station controller (BSC) , which can control several base stations. At step 40 the serving base station BSl includes said time offset in the handoff command to the mobile station MS. Next, at step 42 the mobile station MS shifts to the new frequency and, by using said time offset, shifts its burst structure to allow immediate transmission without further measurements. The shift is performed in the time shifting means TSM described above. At step 44 the handoff of the mobile station MS from the serving base station BSl to the target base station BS2 is performed. The flow chart ends at step 48 which comes after step 44. According to the invention the target base station BS2 can, by monitoring the transmission between the mobile station MS and the serving base station BSl, measure the time offset T which the mobile station MS must shift its transmission phase to fit the timing in the target base station BS2. The time offset T measured includes both the difference in the timing between the serving base station BSl and the target base station BS2 and the difference in transmission delay between the serving connection and the target connection due to difference in distance from the mobile station MS to the respective basestation BSl and BS2. The mobile station MS receives a handoff command comprising the new channel and the measured time offset T. The mobile station also separately receives an estimated value of the time alignment for the new connection. Since the time offset needed for correct burst syncronization of the transmission of the mobile station thus is given, the mobile station MS does not need to perform a resyncronization prior to transmitting any bursts. If the method according to the invention is used in a syncronized system, i.e. base station BSl and BS2 have the same timing, another advantage is that the measured time offset will be the correct difference in time alignment between the current connection and the target connection. Thus, the correct (not estimated) time alignment valuefor the new connection can be calculated. According to the invention, a similar procedure can be used in connection with a set-up of a connection between the mobile station MS and a base station. During such a set-up situation, a handoff of the mobile station MS from a control channel to e.g. a digital traffic channel on one and the same base station could be part of the procedure. As indicated above, such a handoff is one possible type of intra-cell handoff. Also in such a case and for other types of intra-cell handoffs, e.g. an intra-cell handoff between two traffic channels, a time-consuming resynchronization of the mobile station will be avoided. In the above cases of intra-cell handoffs to the same base station, the channels are supposed to be non-synchronized and/or to have a phase difference unknown to the mobile station. The time offset T that controls the transmission phase shift of the mobile station at handoff does not have to be "exact". It is enough if the time offset controls the mobile station good enough in order for the signal of the mobile station not to collide with the signals of other mobile stations and if the time offset makes it possible to control the transmission phase of the mobile station to within the time window where a synchronization word can be found at once. Hence, the determined time offsets have to be quantized. By means of the method and the apparatus according to the invention, the disruption of an ongoing speech in connection with a handoff, will be so short that the disruption may easily be masked by a speech decoder (not shown) , resulting in a more or less seamless handoff not noticeable by the human ear. A disruption of less than 40 ms has been obtained. The term handoff used in this application comprises handoffs from a first channel in a serving base station to a second channel in a target base station, handoffs from a control channel to a traffic channel when setting up a connection between a base station and a mobile station, as well as handoffs between traffic channels in a common base station and other possible handoffs. It is to be understood that the terminology used herein is for the purpose of description and not of limitation.

Claims: CLAIMS 1. In a mobile communication system, a method for handing off a mobile station (MS) from a first channel (CHI) on a serving base station (BSl) to a second channel (CH2) on a target base station (BS2) , said mobile station (MS) transmitting at a first transmission phase on said first channel (CHI) , said second channel (CH2) being ready to take over the communication with said mobile station (MS) transmitting at a second transmission phase, characterized by - prior to a handoff, determining, by tuning in to said mobile station (MS) in said target base station (BS2) , the relative time offset between said first transmission phase and said second transmission phase, and - upon handoff, shifting the transmission phase of said mobile station (MS) by said determined time offset relative to said first transmission phase. 2. Method according to claim 1, characterized in that said mobile station (MS) is informed about said determined time offset or provided with information allowing said mobile station (MS) to establish said determined time offset, prior to a handoff. 3. Method according to claim 2 , characterized in that said mobile station (MS) , upon a handoff, shifts its transmission phase by said determined time offset relative to said first transmission phase. 4. Method according to claim 1, wherein said serving base station and said target base station are one and the same single base station, characterized in that said time offset is determined in said single base station. 5. Method according to claim 1, characterized in that said determined time offset is quantized 6. In a mobile communication system, an apparatus for handing off a mobile station (MS) from a first channel (CHI) on a serving base station (BSl) to a second channel (CH2) on a target base station (BS2) , said mobile station (MS) trans- mitting at a first transmission phase said first channel (CHI) , said second channel (CH2) being ready to take over the communication with said mobile station (MS) transmitting at a second transmission phase, characterized by - a time offset determining means (TODM) , provided in said target base station (BS2) and adapted to tune in to said mobile station (MS) , for determining, prior to a handoff, the relative time offset between said first transmission phase and said second transmission phases and - a time shifting means (TSM) for shifting, upon handoff, the transmission phase of said mobile station (MS) by said determined time offset relative to said first transmission phase. 7. Apparatus according to claim 6, charcterized in that means are provided to inform said mobile station (MS) about said determined time offset or to provide information allowing said mobile station (MS) to establish said determined time offset, prior to a handoff. 8. Apparatus according to claim 7, characterized in that said time shifting means (TSM) is provided in said mobile station (MS) and is adapted, upon a handoff, to shift the transmission phase of the mobile station (MS) by said deter¬ mined time offset relative to said first transmission phase. 9. Apparatus according to claim 6, wherein said serving base station and said target base station are one and the same single base station, characterized in that said time offset determining means is provided in said single base station. 10. Apparatus according to claim 6, characterized in that means are provided to quantize said determined time offset.

Inventors: ANDERSSON CLAES HAAKAN, LUNDQVIST NILS PATRIK, ANDERSSON, CLAES, HAAKAN, LUNDQVIST, NILS, PATRIK
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1994
Main classification: H04B7
IPC codes: H04B7, H04J3, H04W36, H04W56